Thin-film crystalline Si solar cells on cheap foreign substrates are a promising alternative to traditional bulk crystalline silicon solar cells because of their higher potential for cost reduction. However, most of these approaches have led to devices with much lower efficiency than traditional solar cells. A major reason is the lower crystallographic quality of the layers obtained by deposition and/or crystallization.
One approach to crystallization is to use the metal-induced crystallization phenomenon (MIC), also known as metal induced layer exchange phenomenon, to create a thin layer with large grains, typically followed by epitaxial deposition. Examples of this technique are given in for instance S. Gall, J. Schneider, M. Muske, I. Sieber, O. Nast, W. Fuhs, Proceedings PV in Europe (2002) 87; N. P. Harder, J. A. Xia, S. Oelting, O. Nast, P. Widenborg, A. G. Aberle, Proceedings 28th IEEE PVSC (2000); S. Brehme O. Nast, D. Neuhaus, S. Wenham, IEEE trans. el. Dev. 46 (1999) 2062; all of which are incorporated herein by reference in their entirety.
One particular example of MIC is aluminum induced crystallization (AIC). AIC can be summarized as follows. First a metal layer, e.g. an aluminum layer, is deposited on a foreign substrate. Next, the metal layer, e.g. aluminum layer, is oxidized (for instance by exposure to air) to form a thin layer of metal oxide, e.g. aluminum oxide. Then, amorphous silicon is deposited on the metal oxide layer (e.g. an aluminum oxide layer). The sample is then annealed at a temperature below the eutectic point of the silicon-aluminum oxide.
During annealing, a layer exchange takes place: the silicon atoms diffuse into the metal layer (e.g. Al layer) and crystallize together. The metal atoms (e.g. Al atoms) move to the top surface. If the layer exchange process is successful, the final structure consists of a continuous layer of large grain polycrystalline silicon (e.g. typically with grain sizes in the range from 1 μm to 100 μm) on the foreign substrate covered by a metal layer (e.g. an Al layer) with Si enclosures. Typically the metal layer (e.g. Al layer) is then etched away. The resulting silicon layer or seed layer may contain some secondary crystallites that were formed in the top metal layer (e.g. Al layer) during crystallization. Secondary crystallites with vertical side walls and a smooth upper surface are often referred to as ‘islands’.
An additional layer can be deposited epitaxially on the seed layer, reproducing the grain structure of the seed layer. The presence of secondary crystallites and/or islands on the seed layer may have a negative effect on the quality of the layer that is grown epitaxially on top of the seed layer. Therefore, a better epitaxial quality may be obtained if the secondary crystallites and/or islands are removed before epitaxial growth. For example, in Patent Application WO 2004/033769, the islands are removed after having removed the metal layer, e.g. aluminum layer, by a lift-off process. This island removal process is based on the presence of a thin aluminum hydroxide and/or aluminum oxide between the polycrystalline silicon film (basic seed layer) and the islands, and is therefore closely related to the AlC layer formation itself.
If the epitaxy process is done at low temperature (e.g. by using ECR PECVD or Ion-assisted deposition), low-cost glass can be used as a substrate. In the context of solar cell manufacturing, the use of glass substrates could lead to very low cost solar cells. However, low temperature epitaxy on imperfect surfaces presents a serious technological challenge.
Alternatively, the epitaxial growth can be done with a high temperature technique (e.g. thermal chemical vapor deposition). Using a high temperature technique has advantages (good epitaxial quality with simple process), but also imposes restrictions on the possible foreign substrates. Standard low-cost glass cannot be used because it cannot withstand high temperatures. Therefore ceramic substrates are often considered for the high temperature route. Aluminum-induced crystallization on ceramic substrates however results in an average grain size that is low (e.g., about 1 to 2 micron), and a high density of islands.
In “Formation of polycrystalline silicon on foreign substrates by combination of CVD and Aluminum-Induced crystallization techniques”, E Pihan, A. Slaoui, M. Rusu, Conference “PV in Europe”, Rome 2002, which is incorporated herein by reference in its entirety, nucleation and growth of polycrystalline silicon layers on foreign substrates using the aluminum induced crystallization process (AIC) of amorphous silicon is presented. The foreign substrates used are mullite ceramic substrates as well as thermally-oxidized silicon substrates. It is shown that the combination of AIC and thermal CVD can be applied on ceramic substrates, though the quality of the resulting crystalline silicon layer is much lower on the mullite substrates than on the thermally-oxidized silicon substrates, illustratively shown in FIG. 2 of this paper, because the density of grains on mullite substrates is much higher than the density of grains on thermally oxidized silicon substrates, and thus the grain size is much smaller.
Parameters that can define the quality of a crystalline film created by an MIC technique are grain size, continuity of the layer, island density, crystallographic properties (e.g., defects and grain boundaries) and electrical properties.
Crystals are referred to herein based on their grain sizes, as provided in Table 1. The term “crystalline silicon” refers to silicon of all crystal types, but excludes amorphous silicon.
TABLE 1Grain size of silicon crystalNameAtomic sizeAmorphousAtomic size—10 nmNanocrystalline10 nm-1 μmMicrocrystalline1 μm-100 μmPolycrystalline100 μm-1 mmMulticrystallineInfiniteMonocrystalline
While the above-described methods of MIC have produced crystalline silicon on foreign substrates, these techniques have not produced silicon layers with high enough crystalline quality for many uses, such as for use in solar cells. Accordingly, there is an unmet need for an improved method of forming crystalline silicon on foreign substrates.